Joint schemes for the process mean and the variance are essential to determine if unusual variation in the location and spread of a quality characteristic has occurred.
This paper comprises a systematic study on the phenomena of misleading, unambiguous and simultaneous signals while dealing with Shewhart and EWMA joint schemes for the process mean and the variance of a normally distributed quality characteristic.
Examples have been added to illustrate how the R statistical software can be used to assess the performance of joint schemes in practice, namely when it comes to the occurrence of those valid signals.